Centrifugal compressors are required to have high pressure and high efficiency over a wide operating range.
FIG. 7 shows an enlarged sectional view of a substantial part of an upper half of an axis of a rotary shaft of a compressor impeller in a centrifugal compressor.
A compressor 1 of a centrifugal compressor mainly comprises a compressor impeller 3 constituted by a rotating hub 31 and a large number of centrifugal vanes 32 attached to an outer circumferential surface of the hub 31, a shaft 2 coupled to a rotary drive source of the compressor impeller 3, and a compressor housing 11 which houses the compressor impeller 3 and the shaft 2 and which forms a flow path of a fluid.
The compressor housing 11 is provided with a diffuser portion 13 which forms a roughly donut shape on an outer circumferential side of the compressor impeller 3 and which enables recovery of static pressure by decreasing the velocity of fluid that is discharged from the compressor impeller 3, a scroll portion 12 which is formed on an outer circumferential side of the diffuser portion 13 so that a cross-sectional area of the scroll portion 12 spirally increases in a circumferential direction and which collects gas over the entire circumference, and an outlet tube (not shown).
When the compressor impeller 3 rotates, the centrifugal vanes 32 compress a fluid such as a gas or air introduced from an air passageway 15. A flow (fluid) of gas or air or the like formed in this manner proceeds from an outer circumferential end of the compressor impeller 3, passes through the diffuser portion 13 and the scroll portion 12, and is sent out from the outlet tube.
FIG. 8 is a schematic diagram showing an example of the scroll portion 12 in plan view.
The scroll portion 12 has a constant distribution of a radius R (a centroid P0 of a cross section of the cross section 12 and an axis L1 of the shaft 2) for positions determined at intervals of 30 degrees in a clockwise direction from a position of 60 degrees, where an end point (360 degrees in FIG. 8) of the scroll is set as a 0 base.
FIG. 9A shows a constant distribution of a radius R, wherein angle positions in a circumferential direction are plotted on a horizontal axis and the radius R from an axis L1 of a rotary shaft of the compressor of the scroll portion 12 to a centroid P of a scroll cross section is plotted on a vertical axis.
In addition, FIG. 9B is a cross-sectional view in which cross sections at respective circumferential positions (at intervals of 30 degrees) of the scroll portion 12 when a position at 60 degrees in a clockwise direction in FIG. 8 is set as a base are laminated on top of each other, and shows a variation of a centroid P0 of the scroll cross section in a direction of the radius R.
Since fluid (gas) from the compressor impeller 3 flows into the scroll portion 12 via the diffuser portion 13 over approximately the entire circumference of the scroll portion 12, each cross-sectional area of the scroll portion 12 increases at a constant rate χ in a flowing direction of the fluid in accordance with an amount of inflow of the fluid.
Fluid velocity in the scroll becomes constant when equilibrium is established between the enlargement rate χ (constant rate) of the cross-sectional area of the scroll portion 12 and a rate of increase of the amount of fluid inflow into the scroll portion 12 from the diffuser portion 13.
Japanese Patent Application Laid-open No. 2010-209824 (Patent Document 1) discloses conventional art in which a shape of a scroll is varied.
In Patent Document 1, a first transition portion is provided in which a cross-sectional area of a scroll portion comprising a flow path formed in a spiral shape around a rotary shaft of rotor blades of a turbine which produces power by supplying a fluid gas to the rotor blades gradually decreases while a cross-sectional shape of the scroll portion transitions from a square shape with rounded corners to a circular shape, wherein curvature radii of the corner portions of the first transition portion are essentially set to a same magnitude.
In addition, by forming the first transition portion, giving the cross-sectional shape a square shape with rounded corners at phases where the scroll cross section can be increased, and giving the cross-sectional shape a circular shape at phases where the scroll cross section cannot be increased, the technique disclosed enables a sufficient cross-sectional area of the flow path to be secured in each phase and pressure loss of the fluid to be reduced.    Patent Document 1: Japanese Patent Application Laid-open No. 2010-209824
However, the technique according to Patent Document 1 is related to a scroll shape of a turbine which produces power by supplying a fluid gas to rotor blades and expanding the fluid gas, and differs from the present application in which a fluid (gas) is compressed with respect to how a fluid flows and in fluid characteristics.
Therefore, the way scroll shapes are viewed also differs.
In addition, centrifugal compressors are required to have a high pressure ratio and high efficiency over a wide range.
When a fluid flowing out from a compressor has velocity, since dynamic pressure increases but static pressure does not increase, pressure ratio and efficiency decline. Therefore, the velocity must be reduced within the compressor. Although pressure is recovered by reducing the velocity of the fluid in the diffuser portion, the velocity of the fluid differs between an inner side and an outer side of the scroll in a cross section of each portion of the scroll portion, which makes it difficult to accurately determine flow rate and velocity.
In addition, the velocity of the fluid in the scroll portion can conceivably be decreased by linearly increasing (increasing at a constant ratio) a size of cross sections of the scroll portion. In this case, due to constant decrease of velocity in a flow direction, a boundary layer between the fluid and a wall surface of the scroll portion increases and prevents sufficient static pressure recovery, and an occurrence of surging results in defects such as a reduction in an operating range and a decline in turbocharging efficiency.